1. Technical Field
The present invention is directed to cooling assemblies and other apparatus used for removing heat from electronic devices. More particularly, the present invention is directed to an electronic module with integrated thermoelectric cooling elements. Even more particularly, this invention is directed to a multi-stage thermoelectric apparatus employing a thermal space transformer which is interposed within an electronic module in thermal contact with a heat generating component thereof, such as an integrated circuit chip.
2. Description of the Related Art
As is well known, as the circuit density of electronic chip devices increases, there is a correspondingly increasing demand for the removal of heat generated by these devices. The increased heat demand arises both because the circuit devices are packed more closely together and because the circuits themselves are operated at increasingly high clock frequencies. Nonetheless, it is also known that runaway thermal conditions and excessive heat generated by chips is a leading cause for failure of chip devices. Furthermore, it is anticipated that the demand for heat removal. for these devices will increase indefinitely. Accordingly, it is seen that there is a large and significant need to provide useful cooling mechanisms for electronic circuit devices.
Thermoelectric cooling elements operate electronically to produce a cooling effect. By passing a direct current through the legs of a thermoelectric device, a temperature difference is produced across the device which is contrary to that which would be expected from Fourier""s Law.
At one junction of the thermoelectric element both holes and electrons move away, toward the other junction, as a consequence of the current flow through the junction. Holes move through the p-type material and electrons through the n-type material. To compensate for this loss of charge carriers, additional electrons are raised from the valence band to the conduction band to create new pairs of electrons and holes. Since energy is required to do this, heat is absorbed at this junction. Conversely, as an electron drops into a hole at the other junction, its surplus energy is released in the form of heat. This transfer of thermal energy from the cold junction to the hot junction is known as the Peltier effect.
Use of the Peltier effect permits the surfaces attached to a heat source to be maintained at a temperature below that of a surface attached to a heat sink. What these thermoelectric modules provide is the ability to operate the cold side below the ambient temperature of the cooling medium (air or water) or provide greater heat removal capacity for a given cold plate or component temperature. When direct current is passed through these thermoelectric modules a temperature difference is produced with the result that one side is relatively cooler than the other side. These thermoelectric modules are therefore seen to possess a hot side and a cold side, and provide a mechanism for facilitating the transfer of thermal energy from the cold side of the thermoelectric module to the hot side of the module.
Conventional configurations and positionings of thermoelectric assemblies are nonetheless seen herein to be unnecessarily limiting in terms of the thermal energy which may be transferred and the long term reliability attained. Thus, while the use of thermoelectric devices is seen to provide a means for the solid state cooling of adjacent electrical devices, their efficiency and reliability has been less than optimal.
In addition, as complementary metal oxide semiconductor (CMOS) circuit and process technologies approach scaling limits, it becomes necessary to seek approaches and opportunities to achieve further performance gains. One avenue which is receiving increased attention is the operation of CMOS circuits at lower temperatures. The circuit performance enhancements which may be achieved vary from about 1.1xc3x97 at a cooling condition of 25xc2x0 C., to 1.8xc3x97 at a cooling condition of xe2x88x92200xc2x0 C. To obtain cooling conditions down to about xe2x88x9250xc2x0 C. or so, conventional refrigeration technology may be utilized. However, conventional refrigeration systems may be difficult to control for variations in heat load, and may not be responsive enough during transient operating conditions.
Thermoelectric devices, used in conjunction with other module cooling technologies, are known to be able to lower junction temperatures below that which can be achieved by the other module cooling technologies alone. Problems arise, however, when thermoelectric devices are taken down in temperature below the ambient, and in particular, below the environment dew point temperature. Traditionally, thermoelectric devices, which are separate from and attached to an electronic module casing (i.e., cap) are exposed to the system environment. When brought down in temperature below the dew point, condensation forms. This condensation significantly reduces the fatigue life due to corrosion of the solder joints forming the thermoelectric junctions. In fact, the mere presence of oxygen accelerates solder fatigue cracking.
Advantageously, disclosed herein is a means for improving thermoelectric device reliability when used in conjunction with cooling of electronic modules. Specifically, a thermoelectric apparatus is integrated within an electronic module itself so that the thermoelectric apparatus is maintained in a controlled, i.e., oxygen and moisture restricted, environment. Furthermore, by integrating a thermoelectric apparatus into the electronic module, power delivery to the thermoelectric devices can be integrated with the module, thus simplifying system level design, i.e., in comparison with delivering power to externally mounted thermoelectric devices.
To summarize, in one aspect, presented herein is an electronic module which includes an electronic device and a thermoelectric assembly. The thermoelectric assembly is thermally coupled to the electronic device for removing heat generating thereby. A housing surrounds the electronic device and the thermoelectric assembly so that the electronic device and the thermoelectric assembly are sealed from the surrounding environment.
As an enhancement, the thermoelectric assembly can comprise a cascaded thermoelectric cooling assembly with a thermal space transformer interposed between adjacent thermoelectric stages for improved thermal performance. The thermal space transformer provides enhanced spreading of heat being xe2x80x9celectronically pumpedxe2x80x9d from the heat load by a first thermoelectric stage, along with the heat dissipated by the thermoelectric elements of the first stage. This spreading of heat takes place in the plane of the thermal space transformer principally in a direction radially outward from the center of the first stage thermoelectric assembly, i.e., in a direction transverse to the principal heat flow through the first thermoelectric stage. Increased thermal spreading makes it possible to utilize a larger surface area for the second stage encompassing an increased number of thermoelectric couples or elements in the second stage, thereby reducing the amount of heat to be xe2x80x9celectronically pumpedxe2x80x9d by each element. The combined effect of the first and second stage thermoelectric assemblies with the thermal space transformer in between is to accommodate an increased amount of heat to be pumped from the electronic device while maintaining the same temperature difference from the base of the first stage (i.e., Tcold) to the top of the second stage (i.e., Thot), or to allow a greater temperature difference between the two stages for the same heat load (i.e., Tcold).
As a further enhancement, by integrating a cascaded thermoelectric cooling assembly within the electronic module, power delivery to the thermoelectric elements can be simplified by eliminating discrete wiring outside of the module. Wire bond or electrical spring contacts can be employed within the module to couple the thermoelectric stages to appropriate power planes, e.g., disposed within the substrate of the module.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered part of the claimed invention.